In the treatment of orthopedic injuries, diseases or deformities, it is well-known to place artificial implants in a patient's body to correct or improve his or her condition. Implant systems and devices are available to fix bones, muscles, tendons, and/or ligaments together or in a particular spatial relation so as to promote healing. For example, in the spinal field, one type of system for correcting and stabilizing the spine includes a bendable rod, which is preferably bent to correspond to the normal curvature of the spine in the particular region of interest. The rod is engaged to vertebrae along a length of the spinal column by way of a number of fixation elements. The variety of fixation elements configured to engage specific portions of the vertebrae includes hooks configured to engage the vertebral laminae and screws which can be threaded into parts of the vertebral bone. Rods or other similar elements can also be useful in correcting other orthopedic problems.
In several available rod-implantation systems, the rod is loaded into a channel in each fixation element. One example of such a system is the Cotrel-Dubosset/CD Spinal System sold by Sofamor Danek Group, Inc. The CD System includes hooks and bone screws with an "open-back"configuration, in which the fixation elements themselves include a body that defines a slot within which the spinal rod is received. The slot includes a threaded bore into which a threaded plug is engaged to clamp the rod within the body of the fixation element. Details of this technology can be found in U.S. Pat. No. 5,005,562 to Cotrel, the specification of which is hereby incorporated by reference. Other devices are also known which have a similar open-back configuration, such as those disclosed in U.S. Pat. Nos. 5,672,176, 5,725,527, 5,738,685, 5,782,833, and 5,728,098.
One difficulty that has been experienced with open-back configurations of medical devices is that the upright legs or wall sections of the body portion can experience splaying after implantation. For example, in the spinal field, after a rod is placed into the channel in the body portion of a open-back spinal fixation element, a closure or locking element is engaged in the body portion over the rod to clamp it within the channel so that there is no relative movement between the rod and the fixation element. Since no relative motion is possible, stresses placed on the rod after implantation are transmitted via the fixation element to the bone. In some cases, these stresses cause the legs or wall sections of the fixation element on either side of the slot to splay or move away from each other. Significant splaying of the fixation element generally results in its failure, since the closure or locking element can no longer be retained in the fixation element to clamp the rod. When that happens, the rod is free to move with respect to the fixation element, and may become disconnected with the fixation element altogether. In such a case, the therapeutic value of the implant is obviated, and further injury or complications may also result.
To prevent splaying, prior medical devices have included a nut, cap, clamp or similar apparatus to surround and hold the legs of the fixation element together. For example, in U.S. Pat. No. 5,672,176 to Biedermann et al., a rod is placed into a slot in the fixation element, the locking member is engaged with the fixation element to press down via an intermediary part on the rod, and an outer nut is threaded on the outside of the fixation element. Although effective in controlling splaying, these devices have tended to be relatively more expensive and less efficient to implant compared with devices without an outer nut or cap. The outer nut or cap also adds to the profile of the medical device, making the device more difficult to implant in the frequently limited area in which to perform surgery and/or place an implant. A larger implant can also result in a higher risk of residual pain to the patient or potential complications.
There is therefore a need remaining in the industry for medical devices, and particularly orthopedic devices, which minimize the profile and bulk of the components of the device and minimizes the cost and difficulty of using such devices, while still preventing splaying of the fixation elements.